Reinforcing cords that are formed using reinforcing fibers such as glass fibers have been proposed as reinforcing materials for reinforcing rubber products such as rubber belts. The rubber products such as rubber belts, however, are subjected to bending stress repeatedly and thereby the performance thereof tends to deteriorate due to bending fatigue. Accordingly, reinforcing cords to be used for such rubber products are required to have high bending fatigue resistance. Furthermore, a timing belt that is used to drive a camshaft of an internal combustion engine of an automobile is required to have high dimensional stability to maintain the suitable timing. Therefore, reinforcing cords to be used for such rubber products are required to have high tensile strength and tensile elasticity.
In order to obtain a reinforcing cord having both high bending fatigue resistance and good tensile properties (i.e., high tensile strength and high tensile elasticity), cords for rubber reinforcement each having a two-layer structure of a core member (inner layer) and a side member (outer layer) have been proposed.
For example, WO 2007/063686 (Document 1) discloses a reinforcing cord using glass fibers as a core member and a side member. In this cord, the relationship between the number of primary twists in the core member and that in the side member and the relationship between the direction of primary twist in the side member and that of final twist therein (Lang's lay type side member) are limited to achieve high bending fatigue resistance and good tensile properties.
Japanese Patent No. 3864820 (Document 2) discloses a hybrid cord in which a highly elastic fiber strand composed of fibers such as polyparaphenylene benzobisoxazole (PBO) fibers having a high elastic modulus is used as a core member and a glass fiber strand having a lower elastic modulus than the highly elastic fiber strand is used as a side member. Japanese Patent No. 3846236 (Document 3) discloses a hybrid cord in which a glass fiber strand is used as a core member and an aramid fiber strand is used as a side member. WO 2004/090224 (Document 4) discloses a hybrid cord in which a carbon fiber strand is used as a core member and a glass fiber strand is used as a side member. In each of these hybrid cords, the number of primary twists and the number of final twists in the core member and the side member each can be adjusted suitably within a predetermined range of values.
JP 2004-011076 A (Document 5) discloses a reinforcing cord in which PBO fibers, glass fibers, or carbon fibers are used for a core member and glass fibers or PBO fibers are used for a side member. In this reinforcing cord, the direction of primary twist and the direction of final twist in the side member are the same, and the direction of primary twist in the core member and the direction of primary twist in the side member are opposite to each other. This configuration provides the reinforcing cord with high bending fatigue resistance and good tensile properties.
None of the reinforcing cords disclosed in the above documents 1 to 5, however, has both sufficiently high bending fatigue resistance and sufficiently good tensile properties, and therefore further improvement are needed. None of the above documents 1 to 5 discloses specific details, such as the relationship between the number of primary twists and the number of final twists, and the directions of these twists, to achieve both higher bending fatigue resistance and better tensile properties, although they define preferable ranges of values for the number of primary twists, the number of final twists, and the like in the core member and the side member. Accordingly, the reinforcing cord disclosed in each of the above documents 1 to 5 has a problem in that if the number of twists is increased simply, the bending fatigue resistance improves but the tensile elasticity and the tensile strength decrease, and if the number of twists is decreased simply, the tensile elasticity and the tensile strength improve but the bending fatigue resistance decreases.